I have a question about a transistor connection. I have a heating element of 50W, 12V. I realized that in order to operate it I needed a transistor to provide a sufficiently high current. Can someone please help me how I do it exactly

If you apply ohms law to your heating element, you are going to have a little over 4.1A into that heater. It's not that it is huge by any means, unless you pick the wrong components to deal with it and they simply go up in a little stinky smoke cloud. Watch the heat dissipation.

TomIt's not a hobby if you're not having fun doing it. Step back and breathe

Okay, I already realized that I needed to use a MOSFET transistor. If I use five heating elements I have to put a transistor in front of each one? How do I know how much current a transistor is going to provide me? And if I do not want to reach temperatures are too high, let's say 50 degrees? Am I using the resistor? And if so, where in the electric circuit do I place it? ... If I understand correctly, i don't need to change my code, I just do a high / low with digitalwrite?

So you want individual control, ie 4A for each MOSFET, those Sparkfun ones are OK, but overkill onthe 60V rating, 30V MOSFETs have lower on-resistance.

0.05ohm x 4^2 = 0.8W per MOSFET, so a small heatsink is indicated. Get some 10 milliohm 30VMOSFETs and no heatsinking needed.

To switch all of them together is 20A which is beyond those MOSFETs as I said earlier, <=5 milliohm isa good figure for that, a bit harder to find as logic-level, but they exist down to about 2 milliohm lasttime I checked.

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